Radiotherapy and imaging apparatus

ABSTRACT

A radiotherapy system comprises a patient support, moveable along a translation axis, an imaging apparatus, comprising a first magnetic coil and a second magnetic coil, the first and second magnetic coils having a common central axis parallel to the translation axis, and being displaced from one another along the central axis to form a gap therebetween, the imaging apparatus being configured to obtain an image of a patient on the patient support, a source of radiation mounted on a chassis, the chassis being rotatable about the central axis and the source being adapted to emit a beam of radiation through the gap along a beam axis that intersects with the central axis, a multi-leaf collimator comprising a plurality of elongate leaves movable between at least a withdrawn position in which the leaf lies outside the beam, and an extended position in which the leaf projects across the beam, and a radiation detector mounted to the chassis opposite the source, the radiation detector having a plurality of detector elements aligned with the elongate leaves when projected onto an isocentric plane.

FIELD OF THE INVENTION

The present invention relates to radiotherapy apparatus, andparticularly to a radiotherapy apparatus comprising a magnetic resonanceimaging (MRI) apparatus.

BACKGROUND ART

It is known that exposure of human or animal tissue to ionisingradiation will kill the cells thus exposed. This finds application inthe treatment of pathological cells, for example. In order to treattumours deep within the body of the patient, the radiation must howeverpenetrate the healthy tissue in order to irradiate and destroy thepathological cells. In conventional radiation therapy, large volumes ofhealthy tissue can thus be exposed to harmful doses of radiation,resulting in prolonged recovery periods for the patient. It is,therefore, desirable to design a device for treating a patient withionising radiation and treatment protocols so as to expose thepathological tissue to a dose of radiation which will result in thedeath of those cells, whilst keeping the exposure of healthy tissue to aminimum.

Several methods have previously been employed to achieve the desiredpathological cell-destroying exposure whilst keeping the exposure ofhealthy cells to a minimum. Many methods work by directing radiation ata tumour from a number of directions, either simultaneously frommultiple sources or multiple exposures from a single source. Theintensity of radiation emanating from each direction is therefore lessthan would be required to actually destroy cells (although stillsufficient to damage the cells), but where the radiation beams from themultiple directions converge, the intensity of radiation is sufficientto deliver a therapeutic dose. By providing radiation from multipledirections, the amount of radiation delivered to surrounding healthycells can be minimized.

The shape of the beam varies. For single-source devices, cone beamscentred on the isocentre are common, while fan beams are also employed(for example as shown in U.S. Pat. No. 5,317,616).

Of course it is also important that the radiation should be accuratelytargeted on the region that requires treatment. For this reason,patients are required to remain still for the duration of the therapysession, to minimize the risk of damage to healthy tissue surroundingthe target region. However, some movement is inevitable, e.g. throughbreathing, or other involuntary movements.

To overcome this problem, it is known to integrate an image acquisitionsystem with the radiotherapy apparatus, to provide real-time imaging ofthe region and ensure that the radiation emitted by the radiotherapyapparatus tracks any movement of the patient. However, the choice ofimaging system is in general limited by the radiotherapy apparatus inwhich it is installed, and in particular by the geometry. For example,magnetic resonance imaging (MRI) systems require magnetic coils to beplaced around the patient. However, these coils will act to blocktherapeutic radiation from reaching the patient.

What is required is an integrated radiotherapy system that delivershigh-quality in both the imaging and treatment of a patient.

SUMMARY OF THE INVENTION

Currently a number of different devices are used for quality assurance(QA) and in-vivo dosimetry with radiotherapy machines.

Electronic portal imaging device (EPID)—this is used to detect the exitdose from the patient and calculate the dose deposited in the patientusing back projection methods.

CT detector on Tomotherapy—the binary multi-leaf collimator used onTomotherapy is shaped as a narrow slit. The radiation fluence from theslit can be measured using a one-dimensional CT detector which ispositioned on the exit side of patient. The dose deposited in thepatient can be calculated using back projection methods.

Diodes—these are used to measure the entrance or exit dose by attachingthem directly on the patient's surface.

However, none of these systems is ideally suited to use with aradiotherapy system comprising an MRI imaging apparatus. The MRI systemhas a narrow window through which the radiation can pass to reach thetarget. The MRI system has a narrow bore so it is not possible to adjustthe position of the patient so that the target is in the centre of thefield. Furthermore an offset target will require different parts of thewidth of the collimator to be used as the gantry rotates.

The inventors of the present invention have overcome the problemsassociated with conventional integrated radiotherapy and imagingsystems, by providing a radiotherapy MRI system with a multi-leafcollimator (MLC) and a radiation detector having a plurality of detectorelements aligned to the width of the leaves of the MLC when projectedonto the isocentric plane. The radiation detector is not a portalimager, as the MRI system is the imaging system. Rather, the radiationdetector provides QA and in-vivo dosimetry, and therefore need not havea high resolution.

The present invention therefore provides, according to one aspect, aradiotherapy system comprising a patient support, moveable along atranslation axis, an imaging apparatus, comprising a first magnetic coiland a second magnetic coil, the first and second magnetic coils having acommon central axis parallel to the translation axis, and beingdisplaced from one another along the central axis to form a gaptherebetween, the imaging apparatus being configured to obtain an imageof a patient on the patient support, a source of radiation mounted on achassis, the chassis being rotatable about the central axis and thesource being adapted to emit a beam of radiation through the gap along abeam axis that intersects with the central axis, a multi-leaf collimatorcomprising a plurality of elongate leaves movable between at least awithdrawn position in which the leaf lies outside the beam, and anextended position in which the leaf projects across the beam, and aradiation detector mounted to the chassis opposite the source, theradiation detector having a plurality of detector elements aligned withthe elongate leaves when projected onto an isocentric plane.

In an embodiment, the radiation detector comprises a plurality ofdetector elements, columns of which may be aligned with a particularelongate leaf. The columns may be spaced between about 2 mm and about 10mm apart when projected onto an isocentric plane. The radiation detectormay be mounted outside the first and second magnetic coils.

In an embodiment, the system further comprises a multi-leaf collimatorcomprising a plurality of elongate leaves disposed with theirlongitudinal directions substantially aligned with the first directionand movable in that direction between a withdrawn position in which theleaf lies outside the beam, an extended position in which the leafprojects into the beam, and a plurality of intermediate positionstherebetween.

The multi-leaf collimator disclosed above may comprise a respectiveplurality of actuators, for moving the plurality of elongate leaves.

In one embodiment, the chassis is continuously rotatable about thecentral axis. In this embodiment, the patient support may be configuredto move along the translation axis as the chassis rotates about thecentral axis, resulting in a helical radiation delivery pattern. Such apattern is known to produce high quality dose distributions.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the present invention will now be described by way ofexample, with reference to the accompanying figures in which;

FIG. 1 shows a radiotherapy system according to embodiments of thepresent invention.

FIG. 2 is a schematic diagram of aspects of the radiotherapy systemaccording to embodiments of the present invention.

FIG. 3 shows a multi-leaf collimator according to another embodiment ofthe present invention.

FIG. 4 is a cross-section view of the multi-leaf collimator shown inFIG. 3, along the line II.

FIG. 5 shows a view of a multi-leaf collimator in which detectorelements according to the embodiments of the present invention have beensuperimposed. FIG. 6 shows the tip of a conventional leaf used in amulti-leaf collimator.

FIG. 6 shows the tip of a leaf according to an embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG. 1 shows a system according to embodiments of the present invention,comprising a radiotherapy apparatus and a magnetic resonance imaging(MRI) apparatus. The radiotherapy apparatus 6 and MRI apparatus 4 areshown schematically in FIG. 2.

The system includes a couch 10, for supporting a patient in theapparatus. The couch 10 is movable along a horizontal, translation axis(labelled “I”), such that a patient resting on the couch is moved intothe radiotherapy and MRI apparatus. In one embodiment, the couch 10 isrotatable around a central vertical axis of rotation, transverse to thetranslation axis, although this is not illustrated. The couch 10 mayform a cantilever section that projects away from a support structure(not illustrated). In one embodiment, the couch 10 is moved along thetranslation axis relative to the support structure in order to form thecantilever section, i.e. the cantilever section increases in length asthe couch is moved and the lift remains stationary. In anotherembodiment, both the support structure and the couch 10 move along thetranslation axis, such that the cantilever section remains substantiallyconstant in length, as described in our U.S. patent application Ser. No.11/827,320 filed on 11 Jul. 2007.

As mentioned above, the system 2 also comprises an MRI apparatus 4, forproducing real-time imaging of a patient positioned on the couch 10. TheMRI apparatus includes a primary magnet 16 which acts to generate theso-called “primary” magnetic field for magnetic resonance imaging. Thatis, the magnetic field lines generated by operation of the magnet 16 runsubstantially parallel to the central translation axis I. The primarymagnet 16 consists of one or more coils with an axis that runs parallelto the translation axis I. The one or more coils may be a single coil ora plurality of coaxial coils of different diameter, as illustrated. Inone embodiment, the one or more coils in the primary magnet 16 arespaced such that a central window of the magnet 16 is free of coils. Inother embodiments, the coils in the magnet 16 may simply be thin enoughthat they are substantially transparent to radiation of the wavelengthgenerated by the radiotherapy apparatus. The magnet 16 may furthercomprise one or more active shielding coils, which generates a magneticfield outside the magnet 16 of approximately equal magnitude andopposite polarity to the external primary magnetic field. The moresensitive parts of the system 2, such as the accelerator, are positionedin this region outside the magnet 16 where the magnetic field iscancelled, at least to a first order. The MRI apparatus 4 furthercomprises two gradient coils 18, 20, which generate the so-called“gradient” magnetic field that is superposed on the primary magneticfield. These coils 18, 20 generate a gradient in the resultant magneticfield that allows spatial encoding of the protons so that their positioncan be determined from the frequency at which resonance occurs (theLarmor frequency). The gradient coils 18, 20 are positioned around acommon central axis with the primary magnet 16, and are displaced fromone another along that central axis. This displacement creates a gap, orwindow, between the two coils 18, 20. In an embodiment where the primarymagnet 16 also comprises a central window between coils, the two windowsare aligned with one another.

An RF system 22 transmits radio signals at varying frequencies towardsthe patient, and detects the absorption at those frequencies so that thepresence and location of protons in the patient can be determined. TheRF system 22 may include a single coil that both transmits the radiosignals and receives the reflected signals, dedicated transmitting andreceiving coils, or multi-element phased array coils, for example.Control circuitry 24 controls the operation of the various coils 16, 18,20 and the RF system 22, and signal-processing circuitry 26 receives theoutput of the RF system, generating therefrom images of the patientsupported by the couch 10.

As mentioned above, the system 2 further comprises a radiotherapyapparatus 6 which delivers doses of radiation to a patient supported bythe couch 10. The majority of the radiotherapy apparatus 6, including atleast a source of radiation 30 (e.g. an x-ray source) and a multi-leafcollimator (MLC) 32, is mounted on a chassis 28. The chassis 28 iscontinuously rotatable around the couch 10 when it is inserted into thetreatment area, powered by one or more chassis motors 34. In theillustrated embodiment, a radiation detector 36 is also mounted on thechassis 28 opposite the radiation source 30 and with the rotational axisof the chassis positioned between them. The radiotherapy apparatus 6further comprises control circuitry 38, which may be integrated withinthe system 2 shown in FIG. 1 or remote from it, and controls the sourcethe radiation source 30, the MLC 32 and the chassis motor 34.

The radiation source 30 is positioned to emit radiation through thewindow defined by the two gradient coils 18, 20, and also through thewindow defined in the primary magnet 16. According to embodiments of thepresent invention, the source 30 emits so-called “fan beams” ofradiation. The radiation beam is collimated with appropriate shieldingprior to arrival at the MLC 32, by which time it is already“letterbox-shaped” in order to pass through the MLC housing as describedin greater detail below. That is, the radiation beam is relativelynarrow in one dimension parallel to the axis of rotation of the chassis28 (such as 15 cm at a radius of 60 cm), and is relatively wide in adimension that is transverse to the axis of rotation of the chassis.Thus, the beam takes the fan shape that gives it its name. It is thisfan-shaped beam that is ideally suited to the geometry of the system 2,in which two gradient coils 18, 20 are displaced from one another inorder to allow the radiation access to the patient. A fan-shaped beamprovides substantial radiation to the patient through the narrow window,meaning that the gradient coils 18, 20 can be placed closer togetherthan with conventional integrated radiotherapy/imaging systems. Thisallows the gradient coils 18, 20 to generate stronger gradient fieldsthan would otherwise be the case, increasing the quality of the imagesobtained by the MRI apparatus 4. However, the present invention alsocontemplates beams of radiation taking different shapes such as conebeams, etc.

The radiation detector 36 is optimised for the geometry shown in FIG. 1,and can be used for QA and in-vivo dosimetry. The detector 36 ispositioned outside the magnetic coils 16, 18, 20 on the chassis 28,aligned with the radiation beam exit. It therefore has a fixed positionrelative to the radiation source 30 and MLC 32.

Owing to the fact that it is outside the coils 16, 18, 20 the effect ofscattered radiation will be dominated by the materials in the magnet 16,which are a constant and therefore comparatively easy to model. This isunlike existing electronic portal imaging device (EPID) schemes whichare subject to varying scatter from the patient due to differing patientgeometries which is difficult to predict.

Due to the large transverse size of the detector 36, it uses individualdetector elements (not illustrated). These can either be diodes, ionchambers or similar. Because the detector 36 is used only for qualityassurance (QA) and in-vivo dosimetry rather than patient imaging (theMRI apparatus 4 being the primary patient imager), the pitch of thedetecting elements can be relatively coarse, i.e. substantially equal tothe width of the leaves of the MLC 32 when projected onto the isocentricplane. The width of the leaves is defined by the design of the MLC andmay be between 2 mm and 10 mm when projected onto the isocentric plane).

The detector 36 further comprises elements to perform some of themachine QA on the MLC 32, i.e. detecting that the leaves of the MLC arecorrectly positioned. Columns of these detector elements will typicallybe at the pitch of the leaves of the MLC. There may be a number ofcolumns of detector elements to allow the leaves to be detected atdiscrete positions. These columns of detector elements are ideallysuited to an MLC which is fixed in its orientation, because the leaveswill always be aligned to particular columns of detector elements.

In operation, a patient is placed on the couch 10 and the couch isinserted into the treatment area defined by the magnetic coils 16, 18and the chassis 28. The control circuitry 38 controls the radiationsource 30, the MLC 32 and the chassis motor to deliver radiation to thepatient through the window between the coils 16, 18. The controlcircuitry 38 controls the source to deliver radiation in a fan beam, inthe usual pulsed manner. The chassis motor 34 is controlled such thatthe chassis 28 rotates about the patient, meaning the radiation can bedelivered from different directions. The MLC 32 is controlled to takedifferent shapes, thereby altering the shape of the beam as it willreach the patient. Simultaneously with rotation of the chassis 28 aboutthe patient, the couch 10 may be moved along a translation axis into orout of the treatment area (i.e. parallel to the axis of rotation of thechassis). With this simultaneous motion a helical radiation deliverypattern is achieved, known to produce high quality dose distributions.

The MRI apparatus 4, and specifically the signal-processing circuitry26, delivers real-time (or in practice near real-time, after a delay inthe order of milliseconds) images of the patient to the controlcircuitry 38. This information allows the control circuitry to adapt theoperation of the source 30, MLC 32 and/or chassis motor 34, such thatthe radiation delivered to the patient accurately tracks the motion ofthe patient, for example due to breathing.

FIG. 3 shows an MLC 32 according to an embodiment of the presentinvention. In this embodiment, the collimator 32 comprises a housing 46which is effectively shaped as an elongate rectangular aperture. Pairsof leaves (for example as indicated with reference numerals 48 a, 48 b)are located along the housing 46, and are movable into and out of theaperture in a substantially continuous number of positions by action ofa plurality of actuators. The actuators may be operated byelectromagnetic motors, placed outside the coils 16, 18, 20 to minimizeinterference with the magnetic fields present in the MRI apparatus 4. Atone extreme, each leaf may be positioned entirely outside the aperture;at the other, each leaf may be positioned entirely within the aperture.As illustrated, each leaf may be separately controllable to move intoand out of the housing (i.e. the movement of the leaves in each pair isnot linked). This embodiment allows the target region to be tracked moreaccurately, as it does not assume that the target is in the centre ofthe field of view.

The MLC 32 is fixed in its orientation and has a maximum field sizedefined by the shape of the housing 46 that is relatively small in thelongitudinal direction (matched to the width of the gap between the twomagnetic coils 18, 20, typically 15 cm at a radius of 60 cm) andrelatively large in the transverse direction.

In one embodiment, the MLC 32 does not rotate. This makes theconstruction simpler and cheaper. It also makes the geometry more stableand easier to calibrate, and quality assurance easier to achieve. Toovercome this lack of rotation, however, the leaves 48 are thinner thanin conventional MLCs. To prevent these thin leaves from bending undertheir own weight they can be supported across the treatment field. TheMLC 32 comprises a plurality of supports 50 that stretch across thehousing, as shown most clearly in the cross-section view of FIG. 4. Eachleaf 48 is shaped so that a relatively narrow portion slots between apair of adjacent supports 50. Two shoulders are then defined between therelatively narrow region of each leaf and the relatively wide region,which sit on top of the supports 50. The provision of supports 50 ispossible because there is no possibility of a field defining light inthe MRI linac or the use of electrons, as the magnet 16 prevents it. Theadditional material introduced by use of the supports 50 can beinsignificant compared to the material already in the magnet, and sodoes not further interfere with the therapeutic radiation delivered tothe patient. For example, the leaves are typically manufactured from amaterial with a relatively high atomic number (e.g. tungsten) andrepresent a considerable barrier to the radiation due to theirrelatively thick cross section along the beam axis. In contrast, thesupports 50 are relatively thin in that direction, and may be made froma material with a relatively low atomic number (e.g. aluminum). Thecombination of both these factors means that the supports 50 present anegligible barrier to the radiation, even though they are fixed acrossthe beam's path. In alternative embodiments, a sheet of material (e.g.aluminum) may be placed over the exit to the MLC 32, in order to supportthe leaves. The sheet is again thin enough that it does not present asignificant barrier to the radiation. The supports 50 may be grooves inthe sheet, or raised projections extending from the sheet.

Although not illustrated in FIG. 4, the leaves 48 may be thicker inparts further from the source of radiation 30 than parts nearer thesource of radiation. That is, as the radiation beam diverges into thefan shape according to the present invention, so the leaves alsoincrease in width so that the radiation beam is effectively blockedalong the full length of the leaf 48.

The leaves 48 move only in the longitudinal direction. This makes theleaves short as they only have to traverse the small dimension of thecollimator 32. As they are only moving a small distance the tip of eachleaf can have a large radius and thereby minimise the radiationpenumbra. Also, moving in this direction facilitates target tracking astargets generally move due to breathing and this is in a predominantlylongitudinal direction.

FIG. 5 shows a view of the multi-leaf collimator 32 on which detectorelements 52 of the radiation detector 36 have been superimposed. Not allof the leaves 48 or detector elements 52 are shown for clarity. It canbe seen that the detector elements 52 are arranged into columns whichare aligned with the leaves 48 of the multi-leaf collimator.Specifically, a single column of detector elements 52 is aligned with arespective single leaf or single leaf pair 48. This allows detector 36to perform quality assurance on the position of the leaves 48, but isgenerally insufficient for use as an imager of the region that is beingtargeted for therapy.

FIG. 6 shows the tip of a conventional leaf. It can be seen that the tipis relatively rounded. FIG. 6 shows the tip of a leaf according toembodiments of the present invention. The tip has a much larger radius(i.e. is flatter with sharper edges) due to the short range of travel ofthe leaves across the aperture. The latter tip creates a sharperdefinition between areas in which radiation is allowed to pass throughthe MLC 32, and areas where the radiation is blocked. This increases theaccuracy with which radiation can be applied to the patient.

The present invention therefore provides a system which incorporatesboth a radiotherapy apparatus and an MRI apparatus. A multi-leafcollimator is used to collimate and control the radiation beam before itimpacts the patient. A radiation detector mounted opposite the source ofradiation detects the radiation after it has passed through the patient,and may thus be used in quality assurance and in-vivo dosimetry. Thedetector has a resolution that is substantially equal to a width of theleaves in the multi-leaf collimator when projected onto the isocentricplane, as a high resolution is not required owing to the MRI apparatus.

It will of course be understood that many variations may be made to theabove-described embodiment without departing from the scope of thepresent invention.

Although the present invention has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention.

1. A radiotherapy system comprising: a patient support, moveable along atranslation axis; an imaging apparatus, comprising a first magnetic coiland a second magnetic coil, the first and second magnetic coils having acommon central axis parallel to the translation axis, and beingdisplaced from one another along the central axis to form a gaptherebetween, the imaging apparatus being configured to obtain an imageof a patient on the patient support; a source of radiation mounted on achassis, the chassis being rotatable about the central axis and thesource being adapted to emit a beam of radiation through the gap along abeam axis that intersects with the central axis; a multi-leaf collimatorcomprising a plurality of elongate leaves movable between at least awithdrawn position in which the leaf lies outside the beam, and anextended position in which the leaf projects across the beam; and aradiation detector mounted to the chassis opposite the source, theradiation detector having a plurality of detector elements aligned withthe elongate leaves when projected onto an isocentric plane.
 2. Theradiotherapy system as claimed in claim 1, wherein the plurality ofdetector elements are arranged in columns.
 3. The radiotherapy system asclaimed in claim 2, wherein each of the columns of detector elements isaligned with a respective elongate leaf.
 4. The radiotherapy system asclaimed in claim 2, wherein the columns of detector elements are spacedbetween about 2 mm and about 10 mm apart when projected onto anisocentric plane.
 5. The radiotherapy system as claimed in claim 1,wherein the radiation detector is mounted outside the first and secondmagnetic coils.
 6. The radiotherapy system as claimed in claim 1,wherein the multi-leaf collimator further comprises a plurality ofsupports across the beam, for supporting the elongate leaves at least intheir respective extended and intermediate positions.
 7. Theradiotherapy system as claimed in claim 1, wherein the multi-leafcollimator further comprises a respective plurality of pneumatic orhydraulic actuators, for moving the plurality of elongate leaves.
 8. Theradiotherapy system as claimed in claim 1, wherein the elongate leavesof the multi-leaf collimator are further moveable to a plurality ofintermediate positions between their withdrawn and extended positions.9. The radiotherapy system as claimed in claim 1, wherein the multi-leafcollimator is fixed in its rotation with respect to the source ofradiation.
 10. The radiotherapy system as claimed in claim 1, furthercomprising a control means for the source adapted to control the sourceso as to deliver a therapeutic radiation dose to a patient on thepatient support, the control means being adapted to receive magneticresonance images from the imaging apparatus during delivery of the dose.11. The radiotherapy system as claimed in claim 1, in which the chassisis continuously rotatable about the central axis.